How this instrument works
The 3-site skinfold method pinches subcutaneous fat at three locations — chest, abdomen and thigh for men; triceps, suprailiac and thigh for women — sums the three readings in millimetres, and feeds that sum along with age into a sex-specific cubic regression to estimate body density. The Siri equation then converts density into a fat percentage: 495 divided by density, minus 450. Two separate calculations, one arithmetic pipeline.
Jackson and Pollock derived the men's equation in 1978, and Jackson, Pollock and Ward published the women's version in 1980, both fitted against hydrostatic weighing — underwater weighing, the reference standard for body composition at the time. The regressions include a squared term on the skinfold sum, because the relationship between caliper thickness and true fat isn't a straight line, and a linear age term, because body density at a given skinfold reading drifts with age as fat is redistributed internally even when the pinch itself looks unchanged.
Accuracy here depends heavily on the person holding the calipers. Consistent results require finding the exact anatomical site every time, pinching a genuine skinfold without underlying muscle, and reading a properly calibrated instrument — skills a trained technician develops and a first-time self-measurer usually hasn't. The women's equation in particular is noted in the literature as less reliable for women over 40, where the original validation sample had thinner representation.
- Set the Sex toggle — it selects both the regression coefficients and the three measurement sites.
- Enter Age in years.
- Enter the three skinfold readings in millimetres: chest, abdomen and thigh for men; triceps, suprailiac and thigh for women.
- Read Body density and Body fat (%); the working block shows the skinfold sum feeding the cubic regression.
Worked example — a 25-year-old man, sum of 37 mm
Chest, abdomen and thigh pinches read 10, 15 and 12 mm — a sum of 37 mm. Feed that and an age of 25 into the men's cubic: 1.10938 − 0.0008267×37 + 0.0000016×37² − 0.0002574×25 = 1.10938 − 0.030588 + 0.002190 − 0.006435 = 1.074548 g/mL of body density. Run that through Siri's equation: 495 ⁄ 1.074548 − 450 ≈ 460.66 − 450 = 10.66% body fat.
Change nothing but the person: a 45-year-old man with a thicker 60 mm sum (chest 20, abdomen 25, thigh 15) works out to a body density of about 1.0540 g/mL and roughly 19.66% body fat — both the larger skinfold sum and the higher age pushed the estimate upward, exactly as the two negative-and-positive coefficients in the equation are built to do.
Questions
How accurate is the Jackson-Pollock 3-site method?
Reasonably accurate when performed by a trained technician with a calibrated caliper — the original validation work reported errors in the range of about 3 to 4 percentage points against hydrostatic weighing, the underwater method used as the reference standard when these equations were derived. Untrained self-measurement, inconsistent site location, or a cheap caliper can push the error well beyond that band.
Why does the equation include an age term?
Because body density at a given skinfold sum isn't constant across a lifetime — internal fat tends to redistribute with age even when the subcutaneous layer a caliper actually pinches looks similar. Jackson and Pollock's regressions fit a small linear age term alongside the skinfold sum to correct for that drift, which is why two people with identical caliper readings but different ages get slightly different body fat estimates.
Why are the measurement sites different for men and women?
Because fat distributes differently by sex. The men's equation reads chest, abdomen and thigh, sites where male subcutaneous fat concentrates; the women's equation swaps chest for triceps and adds suprailiac, better matching where female subcutaneous fat tends to sit. Jackson, Pollock and Ward refit the entire regression, not just the site list, around a separate female sample in their 1980 paper.
Can I measure my own skinfolds accurately?
It's difficult, and that's the method's real limitation. Consistent results depend on finding the exact anatomical location every time, pinching a true fold without underlying muscle, and reading a properly calibrated caliper — skills that take practice and are hard to apply to your own body, especially at sites like the back of the upper arm. A second, trained person taking the measurements produces far more reliable numbers than self-measurement does.
Is the women's equation as reliable as the men's?
It's noted in the literature as somewhat less reliable, particularly for women over 40, where the original validation sample had thinner representation and where body composition shifts around menopause aren't fully captured by the regression. The equation still functions and gives a reasonable estimate, but treat results for older women with a wider margin of error than for younger women or for men generally.
What is the Siri equation doing at the end of the calculation?
It converts body density, a physical property, into a body fat percentage. Siri's 1961 equation, %BF = 495 ⁄ density − 450, rests on an assumption about the density of fat tissue versus fat-free tissue — a two-compartment model — that holds reasonably well for most healthy adults. The skinfold regressions do the harder job of estimating density from a tape-and-caliper measurement; Siri's formula is simply the last conversion step.
References
- Jackson & Pollock 1978, Br J Nutr — men's body density equations (PubMed)
- Jackson, Pollock & Ward 1980, Med Sci Sports Exerc — women's body density equations (PubMed)
- Siri 1961, reprinted Nutrition 1993 — density-to-fat conversion (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.